1. Field of the Invention
The present invention relates to an exposure apparatus for projecting a mask pattern onto a photosensitive substrate in photolithography processes in the manufacture of semiconductor devices, image pickup devices (CCDs), liquid crystal display devices, and thin film magnetic heads, etc. More specifically, the present invention relates to a scanning type exposure apparatus, such as a projection exposure apparatus of a step-and-scan system, which synchronously scans a mask and a photosensitive substrate using a projection optical system.
2. Discussion of the Related Art
In the manufacture of semiconductor devices, for example, a reduction projection type exposure apparatus (stepper) of step-and-repeat type (one-time exposure type) has often been used to transcribe a pattern on a mask (reticle) onto each shot region of a wafer coated with a photoresist. However, in response to the recent demand for transcribing circuit patterns on a larger area with higher accuracy without placing a heavy burden on the projection optical system, a so-called step-and-scan type projection exposure apparatus has been developed. The step-and-scan type projection exposure apparatus successively transcribes the image of the pattern on the reticule onto each shot region on the wafer by synchronously moving (scanning) the reticle and the wafer with respect to the projection optical system in such a way that a portion of the pattern on the reticle is successively projected on a wafer via the projection optical system.
The previously known aligner, which transfers the entire pattern of the reticle onto the entire wafer with a positive image of equal magnification by a single exposure operation using a single body type stage, is a prototype of the scanning type exposure apparatus. In contrast, in the step-and-scan system, since a reduction-type projection optical system is normally used, it is necessary to drive the reticle stage and wafer stage independently at a speed ratio proportional to the reduction ratio of the projection optical system. Also, since movement between shot regions is performed in a stepping mode, the mechanism of the stage system is complex and requires an extremely high level of control.
The following is one example of a method for controlling the stage system in a conventional projection exposure apparatus of step-and-scan type. After aligning the reticle stage with the wafer stage at the beginning of scanning, the wafer stage and the reticle stage are synchronously moved (scanned) at a predetermined respective speeds in predetermined directions. During such scanning operation, the positional deviations in the scanning direction and non-scanning direction (direction perpendicular to the scanning direction) of both stages are monitored using laser interferometers and the positional deviations thus measured are offset by a servo control system. In other words, in the conventional method, these positional deviations are corrected by feeding back the measured positional deviations in a closed feedback loop.
As described above, in the conventional step-and-scan type projection exposure apparatus, the positional deviations in the scanning direction and non-scanning direction of the reticle stage and wafer stage are reduced by feeding back the positional deviations to the servo control system during scanning exposure. However, since there is a limit in improving the response speed of this type of feedback system operating in response to the positional deviation amount, some follow-up errors (positional deviation) remain.
More specifically, even if the scanning directions of the wafer stage and the reticle stage are designed to be parallel to each other, some angular errors may exist between these scanning directions due to mechanical assembly errors, or the like. When such angular errors exist, a positional deviation of the reticle stage relative to the wafer stage is generated in the non-scanning direction during scanning exposure. Using feedback control, a force is generated to correct this positional deviation. Nevertheless, since there is a limit in the response speed of the feedback system, a certain degree of follow-up error will always exist in the non-scanning direction.
One way to reduce the follow-up errors of this type is to increase the gain of the feedback system. However, if the gain of the feedback system is increased, mechanical resonance can be easily excited in the stage system, and accordingly the stage system may vibrate during the scanning exposure.